Noise boosts nanotube antennas

By
Eric Smalley,
Technology Research NewsResearchers at the University of Southern
California have shown that the right amount of noise can enable carbon
nanotube transistors to detect weak electrical signals. This is the same
effect -- stochastic resonance -- that neurons use to communicate in biological
brains.

Systems that exhibit this effect have thresholds, meaning signals
must exceed a minimum strength in order for a transistor or neuron to
detect them. Adding noise raises the overall energy level, boosting signals
that are otherwise too weak to cross the threshold. Too little noise has
no effect; too much swamps the signal.

The effect had been predicted in carbon nanotubes because transistors
made from the tubes have a threshold. The researchers confirmed that nanotube
transistors can detect subthreshold electrical signals under electrically
noisy conditions.

Antennas made from arrays of nanotubes would be particularly useful
for spread spectrum communications, which involve distributing communications
signals across a broad range of frequencies in order to improve reception
and make eavesdropping more difficult. "Each tube can act as a dedicated
antenna or signal detector for a specific signal," said Bart Kosko, a
professor of electrical engineering at the University of Southern California.

In theory, billions of nanotubes could be packed into individual
chips. "One scheme would allow each tube to decode for a given frequency,"
said Kosko.

A major challenge in carrying out the scheme is finding efficient
ways to tune the large numbers of nanotubes, said Kosko. A carbon nanotube's
resonant frequency depends on its length.

The stochastic resonance effect also makes carbon nanotubes good
candidates for processing pixel-based image data. A nanotube transistor
could drive each pixel in a display, and noise could help the transistors
pick up subthreshold signals, making the transistors more sensitive to
input, which would improve image quality.

Nanotube transistor arrays could also be used to sense chemical
and biological substances. Carbon nanotube transistors work in saline
solution, which means the devices could work inside the body, said Kosko.
"A more speculative application would be inserting nanotube arrays into
damage nerve tissue to detect nerve signal spikes," he said.

The researchers' experiment involved sending subthreshold signals
to carbon nanotube transistors and then adding three different types of
noise to the transistor's input. They measured the output to determine
how much of the input signal the transistor detected. Adding noise increased
nanotubes sensitivity in all three cases.

The experiment showed that stochastic resonance in carbon nanotubes
could be extremely useful in signal detection and processing under electrically
noisy conditions, said Deepak Srivastava, a senior scientist and task
lead at NASA Ames Research Center. "Electrically noisy environments can
arise in a variety of conditions such as vast parallel arrays of densely
packed nanotube-based devices, broadband communications systems and chemical
detection in liquid or bio[logical] environments," he said.

The most exciting aspect of the stochastic resonance behavior
is its similarity to biological neurons, said Srivastava. "This shows
that it may be feasible to conceptualize and implement signal pulse-train
type neural networks on carbon nanotube-based branching networks," he
said.

The effect should also apply to non-carbon nanotubes, said Kosko.

Practical applications could be possible in five to ten years,
according to Kosko. Researchers will need to improve nanotube transistor
performance, and develop techniques for a physically and/or chemically
modifying individual tubes in arrays of millions of tubes, according to
Kosko.

Kosko's research colleagues were Ian Y. Lee and Chongwu Zhou.
The work appeared in the December 2003 issue of Nano Letters. The
research was funded by the National Science Foundation (NSF).